After another 10 hours of coding I want to report some good progress:
I've got the inter processor communication working. This was tricky because the F4 is the SPI master and the F1 raises an interrupt wire when any of the switches or encoders change state. For the F4 this transfer is transparent, everything is done by the DMA at both ends and after the 32bit transfer it can process the new input whenever it likes. There are also some commands which change parameters like the max volume and interrogation commands like the firmware version. So the F1 must start a DMA receive but if a key/encoder is pressed it must abort the current DMA (because the first word is already loaded in the SPI register) and start a new one with the new info. This allows the information exchange in just one 32bit transfer.
My encoders can send +/- 7 steps but regardless of how fast I turn them the max was +/- 2 steps. Also the detection algorithm for the encoders should be improved (now I'm basing this on just one sample) because I get the occasional step in the opposite direction.
Also my note generation works. I get sine waves on all outputs but I have a horrible super-imposed noise in my headphone amplifier. I have to admit that I haven't put a scope yet on my boards and my only detection method was a pair of headphones. I can also hear the microcontroller running in my headphones so this is bad. I hope that the only culprit is the headphones amp which was a bit of an afterthought. This is powered by the general 5V line and connected to the digital ground (instead of the analog ground as the rest of the audio path) while it has a 39db gain! I might have to scrap this and use only the jack outputs.
The wave generation is using 48KHz sample rate on all 4 channels and all transfers are DMA. I get a half way transfer interrupt and I'm filling the other half of the buffer based on a precomputed sine wave table and the desired frequency.
What's not working is the digital volume (haven't investigated why), haven't checked the purity of the sine waves with a scope and I get a very rare hangup of the surface processor so there's still a bug in there. Another bug is also lurking in my F4 code which manifest itself only when I'm compiling as a release, the debug version works fine. So programming still required.
All in all I'm very pleased so far because all the audio chains are working at least up the output of the DACs and the processor spends most of the time sleeping in the main loop.
